1. Field of the Invention
The present invention relates to an image processing apparatus and method and, more particularly, to an image processing apparatus and method for converting a signal in a given color space into that in another color space.
2. Description of Related Art
As personal computers and workstations have prevailed, desktop publishing (DTP) and computer-aided design (CAD) are quite common these days. In such situation, a color reproduction technique for reproducing color expressed on a monitor by a computer using color ink in practice is important. For example, a color reproduction technique for faithfully reproducing color, which is expressed on a monitor, by a color printer in case of DTP, and a color reproduction technique for faithfully reproducing color, which is expressed on a monitor, by mixing inks in case of CAD are required.
In such color reproduction techniques, it is a common practice to use a color conversion technique using a uniform colorimetric system as a medium. Taking DTP as an example, an RGB color signal as color expression in a computer is converted into an L*a*b* color signal using a color conversion formula specified by, e.g., the CIE. Furthermore, the L*a*b* color signal is converted into a CMY color signal. A color printer which uses cyan, magenta, and yellow inks prints these color inks at a ratio according to the CMY color signal.
The core technique in this color reproduction technique is a color signal conversion technique for converting, e.g., an L*a*b* color signal into a CMY color signal.
It is relatively easy to implement color conversion from a CMY color signal into an L*a*b* color signal, and as a method of implementing that color conversion, interpolation using a look-up table (LUT) is known. More specifically, print colors output based on CMY signals are actually sampled at given sampling intervals in the CMY color space and undergo colorimetry. In this case, a uniform colorimetric system such as an L*a*b* colorimetric system is used as that upon colorimetry. After that, interpolation computations are made using data at the sampling points to implement color conversion from a CMY color signal into an L*a*b* color signal. The LUT is used as means for holding L*a*b* color signals at the respective sampling points.
On the other hand, it is not easy to obtain conversion formulas for converting an L*a*b* color signal into a CMY color signal. Hence, color signal conversion is implemented by searching for a CMY color signal corresponding to a L*a*b* color signal that can provide a desired print result. As an example of color signal conversion using a search method, a search method using a gradient method or the like is available. However, when such search method is used, if low-accuracy color conversion is implemented, time cost can be suppressed. However, high time cost is required to implement high-accuracy color conversion.
In a certain color matching process or print color process, if only a color signal conversion apparatus from a first color space into a second color space is available, inverse conversion of a color signal in the second color space into that in the first color space is required using that color signal conversion apparatus.
In order to meet such requirement, data obtained by converting color signals having given spatial intervals in the first color space into those in the second color space are prepared in advance using the color signal conversion apparatus. Next, a color signal closest to that in the second color space to be inversely converted in terms of a given measure of distance is selected from the prepared data, and a color signal in the first color space as a conversion source of the selected color signal is determined as an inverse conversion result of the color signal in the second color space to be inversely converted.
However, the inverse conversion result is an approximate value, and the conversion accuracy depends on the spatial intervals of color signals used upon preparing inverse conversion data. Also, high cost is required for inverse conversion in terms of time, and the efficiency is poor.
DTP and CAD works normally repeat image output and image correction a plurality of number of times. Hence, when a high-accuracy color conversion process is used in color signal conversion, satisfactory color reproduction can be assured, but a very long operation time is required. On the other hand, when a low-accuracy color conversion process is used in color signal conversion, color reproduction is not always satisfactory, but the operation time can be shortened.
However, since the color signal conversion technique used in a given apparatus that executes a color signal conversion process is fixed, it is hard to shorten the operation time with an apparatus that executes a high-accuracy color conversion process, and it is hard to obtain a satisfactory color reproduction result with an apparatus that executes a low-accuracy color conversion process.
The present invention has been made to solve the aforementioned problems, and has as its object to provide an image processing apparatus and method, which can convert a signal in a given color space into that in another color space with high accuracy and within a short period of time, without preparing any inverse conversion data.
In order to achieve the above object, a preferred embodiment of the present invention discloses an image processing apparatus for converting an input signal in first color space into an output signal in second color space which is different from the first color space, said apparatus comprising: converting means for converting a provided signal in the second color space into a signal in the first color space; extracting means for extracting a difference between the signal output by said converting means and the input signal; and searching means for searching a signal to be provided to said converting means to minimize the extracted difference.
Also, a preferred embodiment of the present invention discloses an image processing apparatus for converting an input signal in first color space into an output signal in second color space which is different from the first color space, said apparatus comprising: converting means for converting a provided signal in the second color space into a signal in the first color space; extracting means for extracting a difference between the signal output by said converting means and the input signal; first searching means for searching a signal to be provided to said converting means to minimize the extracted difference; and second searching means for searching a signal in the vicinity of the signal searched by said first searching means to minimize the extracted difference.
It is another object of the present invention to provide an image processing apparatus and method which can select a satisfactory color reproduction result or a short processing time.
In order to achieve the above object, a preferred embodiment of the present invention discloses an image processing apparatus for converting an input signal in a first color system which expresses color to be reproduced into an output signal in a second color system which is different from the first color system, said apparatus comprising: plurality of converting means, each of which converts the input signal to the output signal by using a different conversion method; and selecting means for manually selecting one of said plurality of converting means.
It is still another object of the present invention to provide an image processing apparatus and method, which can achieve both a satisfactory color reproduction result and a short processing time.
In order to achieve the above object, a preferred embodiment of the present invention discloses an image processing apparatus for converting an input signal in a first color system which expresses color to be reproduced into an output signal in a second color system which is different from the first color system, said apparatus comprising: plurality of converting means, each of which converts the input signal to the output signal by using a different conversion method; setting means for setting at least one color in the first color system; and selecting means for selecting one of said plurality of converting means based on the input signal and the at least one color set by said setting means.